A liquid crystal display (LCD) consists of an optically active liquid contained within a cavity formed by two glass substrates held in close proximity. By applying an electrical signal thereto, the optical properties of the liquid crystal are altered, and this forms the well-known basis of the display action. There are a variety of techniques for applying and patterning suitable transparent electrode materials, along with the other thin-film coatings necessary for inducing proper operation of such devices, which are known in the art and will not be restated here. Such well known techniques are described in, for example, Badahur, Liquid Crystal Displays (Molecular Crystals & Liquid Crystals 109, 1 1984!), which is incorporated herein by reference.
LCD substrates are typically joined using an adhesive seal at the perimeter of the display, after which the resulting cavity is filled with liquid and sealed to prevent contamination or leakage. The adhesive seal joins the substrates and defines a cavity in the space between them. This spacing determines the thickness of the liquid layer, and tight spacing control is critical to producing high-performance liquid crystal displays.
Thermal stresses arise because the thermal expansion of the liquid is several hundred times that of glass, leading to hydrostatic pressure when the display is thermally cycled. At high temperatures, the expanding fluid generates large pressures within the cell cavity. Since liquid crystal fluids are nearly incompressible, this expansion leads either to a deformation of the cavity, if the substrates are sufficiently flexible, or a failure of the adhesive seal if they are not. Similarly, when it is cooled, the liquid contracts, which can lead to cavitation, i.e. bubble formation.
It is common practice to construct displays using relatively thin glass substrates (0.5-1.1 mm) to provide the required flexure. Note that the spacing of the cell inevitably changes with temperature, which leads to diminished display performance. In general, cell spacing is not uniform across the display aperture when the cell is expanded or contracted.
Sometimes, thick substrates must be used. For example, when LCDs are used as precision optical components, substrates of 3 mm thickness or more are employed, to achieve good flatness (.lambda./8 or better) across the display aperture.
Equivalently, displays are sometimes made from thin substrates to which thick, flat windows are bonded with optical epoxy. The incompressible nature of the fluid means that thermal expansion distorts thick substrates just as much as thin substrates. The main effect of greater substrate thickness is that higher pressures are developed within the fluid cavity. Accordingly, such a device has two problems: the adhesive edge seal is more likely to fail under the higher pressures developed, and the goal of achieving a precise optical figure is thwarted by the inexorable expansion of the fluid.
The same situation occurs when a liquid-based display cell is glued or bonded into an assembly with other cells or additional components. Even if the display itself is stress-relieved by use of thin substrates and avoidance of rigid spacer material, the device is rendered mechanically stiff by means of the other components it is bonded to. This stiffening frequently leads to failure of such assemblies, even though the components are reliable on their own, by rupture of the adhesive edge-seal.
In U.S. Pat. No. 4,310,220, Kuwagaki addresses this problem by incorporating a bubble of gas such as N.sub.2 within the cavity region. To be acceptable, such a bubble must be excluded from the display viewing region, as it would constitute a cosmetic defect. This is a severe limit to Kuwagaki's method, as the presence of a stable bubble implies that there is dissolved gas in the liquid, in equilibrium with the bubble at near-atmospheric pressures. Since common atmospheric gases such as N.sub.2 are soluble in liquid crystal materials and electrochromic electrolytes, this means the liquid must contain dissolved gas in significant quantities, which can lead to bubble formation at unpredictable locations in the display.
Burns, in U.S. Pat. No. 3,771,855, teaches the use of a glass washer and ultrathin (microsheet) glass disk to seal a liquid crystal cell. The glass washer is used as a spacer, to which the microsheet disk is epoxied. Sealing is carried out so as to leave a small gas bubble in the fill hole. Thermal stress relief is said to be provided by flexure of the seal members, although in practice the majority of the stress relief is due to the compliant gas bubble. This may be verified by direct calculation of the compliance of the bubble and of the seal members. In actual practice this method is quite similar to that of Kuwagaki. However, for the reasons just given, such a bubble is to be avoided in LCD displays, and an improved method of stress relief is desirable.
In U.S. Pat. No. 4,256,382, Piliavin et. al. describe a liquid crystal device which includes a silicon membrane layer between the substrates, patterned using semiconductor fabrication techniques. The layer purports to match the thermal expansion of liquid crystalline materials, and provide control of the spacing. Beyond the problem of the expense of such a silicon member, this invention suffers many limitations. For example, silicon has a thermal expansion coefficient only a small fraction that of liquid crystal fluid materials. Thus no meaningful thermal matching is provided. Another purported benefit is that the patterned membrane excludes liquid crystal material from optically unused areas, reducing the volume of expanding material. However, since silicon is nearly incompressible, no benefit accrues. Similar detrimental high pressures are developed, albeit in a reduced volume. Displays made in accordance with Piliavin's invention will not have reduced thermal stress, relative to displays without the spacer membrane, and will be prone to cavitation at low temperatures due to the stiffness of the spacing control.
Another approach is taken by Kirkman et. al. in U.S. Pat. No. 4,545,650, who describe a liquid eletro-chromic (LEC) cell assembly with a total of at least thirteen separate parts, several of which have complex shapes and require machining and/or close-tolerance molding to produce. It uses a fluorocarbon polymer membrane to provide a flexible means to accommodate the thermal expansion and contraction of the liquid. This membrane is installed after filling the display with fluid, and is held in place by means of an annular washer and eccentric cam clamping pin. Aside from the cost involved, this system is bulky and is incompatible with industry-standard techniques for filling and sealing displays.
In U.S. Pat. No. 4,832,460, Fujimura et. al. teach a method for stress-relieving a liquid crystal cell comprising a linear array of optical shutters. Such cells are generally long and narrow, so the substrates form a stiff enclosure which cannot deform to accommodate volumetric changes in the fluid, causing undesirable areas of high localized pressure in the device. Fujimura describes how a cell can be made with additional liquid crystal regions formed by the substrates, not used optically but connected to the display aperture region, which act as pressure relief chambers for the fluid. These regions are much larger and wider relative to, and accordingly by virtue of their much larger relative size are more flexible than, the small and narrow shutter regions. The large pressure relief chambers are thus able to deform to a greater extent than the shutter region to accommodate volumetric changes which may arise in the liquid crystal material. Such a method only works when uniformly thin substrates are used, typically 1.1 mm or less, and the difference in size between the display aperture region and pressure relief region is great. Also, if several such elements are stacked together to form a multi-layer assembly, adjacent elements contact one another and pressure relief is defeated, since such contact prohibits necessary expansion of the pressure relief region.
Thus there exists a need for a device which solves the aforementioned shortcomings of the prior art.